The invention relates to a method of furnishing a substrate with a patterned film of electrode material, whereby use is made of a laser beam.
The invention also relates to a substrate thus furnished with a patterned film of electrode material.
The invention further relates to a display device comprising such a substrate.
Many modern devices incorporate substrates which are provided with a complex patterned film of electrically conductive material. For example, devices such as liquid crystal displays, electroluminescent displays and solar panels commonly employ transparent substrates which are furnished with a precisely patterned film of transparent electrically conductive material, serving as an electrode network. Such films are often provided using etch techniques, whereby a uniform film is first provided on a given substrate and then selectively etched away in accordance with a desired pattern. The nature of such etching processes may be chemical (employing, for example, acid mixtures), physical (using, for example, laser radiation), or a combination of both (such as with laser-induced chemical etching).
A method as described in the opening paragraph, together with a patterned substrate coating as elucidated in the second paragraph, are discussed in an article by Lunney et al. in Appl. Phys. Lett. 59 (6), pp 647-649 (1991). In the disclosed method, films of electrically conductive fluorine-doped tin oxide (430 nm thick) and indium tin oxide (150 nm thick) are directly deposited on separate glass substrates and then selectively etched away using an excimer laser (wavelength 248 nm, pulse length 23 ns). The employed laser beam is used to rapidly (locally) heat the oxide film to a temperature above its boiling point (order of magnitude 2000xc2x0 C.), consequent upon which the film locally vaporises and detaches from the substrate surface.
This known method has, however, a number of attendant disadvantages, some of which can be summarised as follows:
(1) Because the electrode film must be heated to such high temperatures (order of magnitude 2000xc2x0 C.), a relatively high laser fluence is required, typical values being of the order of 1-3 J cmxe2x88x922. In practice, this necessitates either the use of a very powerful laser or the use of a relatively low-power laser with a sharply focused beam. In the latter instance, which is more common, the restricted beam diameter will place an upper limit on the width of each film tract which can be irradiated in a single pass of the laser beam. Etching of large areas of the electrode material will therefore require a time-consuming scanning procedure with the focused laser;
(2) Redeposition of condensed electrode material vapour onto etched areas of the substrate is a common and serious problem. Measures aimed at removing vaporised material before it gets a chance to cool and re-attach itself to the substrate are complicated, expensive, and not entirely effective. For example, even the application of a supersonic gas stream to rapidly blow vaporised material out of the vicinity of the substrate cannot satisfactorily prevent redeposition of electrode material. Such redeposited material must therefore be removed in a supplementary etching step, which is not only time-consuming but can also cause damage to the substrate surface;
(3) Although the known laser etching process generally gives sharper pattern definition than a chemical etching process, there is nevertheless considerable remaining roughness of the various pattern edges and walls. This places definite limits on the fine resolution levels attainable with the known method.
It is an object of the invention to provide a laser etching method which allows the satisfactory use of relatively low laser fluences. It is a further object of the invention that such a method should incur a relatively low attendant risk of redeposition of etch debris. Moreover, the invention aims at the attainment of a relatively high etch definition with such a method.
These and other objects are achieved in a method as described in the opening paragraph, characterised in that a stack is made by providing a surface of the substrate at least with a layer of an assistant material and an overlying layer of the said electrode material, which assistant material is capable of decomposition upon heating with the aid of the said laser beam, subsequent to which the stack is, in accordance with a desired pattern, locally irradiated with the laser beam so as to heat the assistant material to at least its decomposition temperature, consequent upon which the locally overlying electrode material is caused to detach.
The term xe2x80x9cdecompositionxe2x80x9d as here employed should be broadly interpreted as referring to such processes as, for example, chemical decomposition of an explosive substance, rapid boiling or sublimation of a substance having a high vapour pressure, fast emission of hot combustion gases, rapid thermal expansion of product gases produced in a chemical or physical reaction, etc., all of which result in the release of relatively large amounts of energy over a short timescale, and all of which can be induced by sudden thermal excitation (laser beam).
Such decomposition of the assistant material can be incurred by irradiating the stack with the chosen laser beam. Depending on the choice of substrate and electrode materials, and on the fluence of the employed laser, such irradiation of the stack can occur either via the substrate or via the layer of electrode material. If all materials between the laser and the assistant material are transparent to the employed laser wavelength, then the assistant material will be directly heated by radiative laser energy. However, if at least one of the materials between the laser and the assistant material is opaque to the employed laser wavelength, then the assistant material will be indirectly heated by conducted thermal energy.
The mechanism by which the electrode material is caused to detach from the underlying surface in the present invention is thus fundamentally different from the known mechanism employed in the above-discussed state of the art. Rather than being directly vaporised from the substrate surface, as in the known method, the electrode material is instead forcefully detached from the underlying assistant material as a result of that underlying material""s laser-induced decomposition. Since the temperature at which the chosen assistant material decomposes can be very substantially lower than the boiling point of the given electrode material, the inventive method can be satisfactorily and advantageously enacted with a very much lower laser fluence than is necessary in the case of the known method, provided the specific heat capacity of the assistant material is less than or roughly of the same order of magnitude as that of the electrode material.
In numerous appraisal tests of the inventive method, it was repeatedly observed that electrode material removed from the underlying assistant layer was very finely pulverised. The particles of detached electrode material were in fact so small that they tended to remain suspended in the atmosphere above the etched surface (analogous to smoke particles). As a result, redeposition of etched electrode material was not observed, and suspended pulverised material could easily be blown out of the vicinity of the substrate using a common (subsonic) gas stream.
It is a surprising aspect of the inventive method that, despite the considerable energy involved in the decomposition of the assistant material, the topographical and geometrical etching definition obtained with the inventive method is actually extremely sharp, and represents a considerable improvement with respect to the etching definition typically obtainable with the method according to the above-elucidated state of the art.
Because the inventive method requires a relatively low laser fluence, it lends itself particularly to the rapid etching of relatively large patterns. Such patterns can, of course, be etched simply by running a focused laser spot along pre-selected routes on the stack, but such a procedure is very time-consuming for relatively large film areas. An alternative method, which is far quicker, is to project and focus a broadened laser beam through a pre-patterned stencil plate onto (large areas of) the entire stack, thereby simultaneously heating all regions of the assistant material which are to be decomposed. Broadening a given laser beam in this way decreases the imparted laser fluence in the focal plane, but this decrease is compensated by the intrinsically lower laser fluence necessitated by the inventive method in the first place. Although one might expect diffraction effects to adversely affect the quality of the etching definition in the case of such a wide-projection procedure, experiments have shown that the sharpness of patterns etched in this manner is nevertheless highly satisfactory.